CA1067519A

CA1067519A - Process for preparing diphenylamine

Info

Publication number: CA1067519A
Application number: CA252,188A
Authority: CA
Inventors: Hans-Michael Fischler; Dieter Bauer; Hein Quast
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1975-05-13
Filing date: 1976-05-11
Publication date: 1979-12-04
Also published as: CH608781A5; ZA762832B; DE2521293B2; GB1483173A; FR2310997A1; IT1061268B; JPS51138628A; DE2521293A1; BE841762A; JPS584706B2; LU74916A1; ES447825A1; DE2521293C3; FR2310997B1; NL7605129A; BR7602968A

Abstract

PROCESS FOR PREPARING DIPHENYLAMINE

Abstract of the Disclosure Diphenylamine is made by heating aniline under pressure and temperatures of 250-400°C in the presence of a boron/fluorine compound and water.

Description

lOtj'~S~9 BACKGROUND
This invention relates to a process for the preparar tion of diphenylamine from ~niline in the liquid pha~e, in the presence of boron/fluorine compound~ and w~ter.
A process for the preparation of diphenylamine by heating aniline with small amounts of boron fluoride, or of an aniline/boron fluoride or ammonia/boron ~luoride complex, under pressure to temperatures between 250 and 400C i8 described in German Fublished Specification 1,116,231.
However, with this process the reaction must be carried out with anhydrous aniline and, in addition, recovery of the cataly~t involves considerable e~fort.
Furthermore, a process for the preparation of diphenylamine by heating aniline under elevated pressure in the presence of boron fluoride as the catalyst is known from U.S. Patent 3,o71,619 and in this process boron Muoride is preferably employed as ammonium fluoborate.
The disadvantage of this proces~ lies in the lnsolubility o~
the solid catalyst, that is to say ammonium ~luoborate, ~n the reaction mixture and the metering and separating problems associated therewith.

SUMMARY

It has now been found that the abovementioneddiS-; advantages may be completely avoided and, furthermore, better j space-time yields may be obtained with the same amounts of ;~ catalyst when, in place of the ~nhydrou~ boron/fluorine com-pounds used hitherto, water-contsining mixtures of boron/
fluorine compounds are employed as cataly~t~.
According to the present invention there is provided a process for the preparation of diphenylamine by heating aniline under pres ure to temperatures o~
250 to 400C, in the presence of a boron/fluorine compound and water.
Le A 16 373 - 2 -,, `` 106';~519 DESCRIPTION OF THE DRAWING
The acco~panying drawing is a graph comparing the results of Examples 22 - 24 herein.
DESCRIPTION
In order to obtain a good space-time yield, the process according to the invention is ~enerally carried out in the customary manner at elevated temperature and ele~ated pressure, whilst boiling the reaction mixture, and, advantageously, the ammonia gas formed is allowed to escape, at the rate at which it is formed, through a cooled column via a pressure valve and the pressure is thus controlled. Since the reaction is usually carried out at the boiling point of the reaction mixture under the reaction pressure, the pressure and the temperature are interdependent and in some cases are subject to an upper limit imposed by the maximum pressure permissible for the apparatus used or by the expense of appropriate pressure vessels.
In general, therefore, the process according to the invention is carTied out in the temperature range between 250 and 400C under pressures of -~ between 6 and 35 bars; preferably, the reaction is carried out in the temperature range between 300 and 360C and, when the reaction mixture is at i the boil, pressures between 12 and 30 bars are set up when the reaction is ~, carried out at approximately the boiling pressure of the reaction mixture, the composition of which changes during the reaction.
Virtually all boron/fluorine compounds, both in bulk and in the form of their solutions, preferably in water and/or aniline, can be used in the process according to the invention; examples which may be mentioned are gaseous BP3, the known complexes of borontrifluoride with ammonia and organic amines BF3-NH3, BF3 NlcH3)2~ BF3-N(c2H5)2 and BF3-aniline, the BF3-hydrates BF3-H20 and BP3-2H20, BP3-ether complexes like BF3-0(CH3)2 and BF3-0(C2H5)2, -~ solid NH4BF4, commercial solutions of HBF4 or the liquid complex mixtures .,~ .
which are ob~ained .~.

. - 3 -... .
~ .
,.
~ ., .

,; ;". "A "" ,,,",,", j ~ ,~ " - : , , ~ , 106~519 by mixing solid boric acid (or boron trioxide) with solid NH4F, or solid (NH4)HF2 (cf Kirk-Othmer, 2nd ed., Vol. 9, p. 558) ~nd the mixtures of boric acid with hydrofluoric acid in aniline.
Only the boron/fluorine complex compounds containing metal cations are not suitable.
The amount of the boron/fluorine compounds used according to the invention is variable within wide limits.
Appropriately, 0.1 to 5.0, preferably 0.2 to 3.0, % by weight of the boron/fluoriDe compound, calculated as BF3 and based on the amount of aniline employed, is used.
~hen preparing the mixtures of boron/fluorine com-pounds, according to the invention, it is not necessary to maintain a molar ratio of boron : fluorine of 1 : 3. The ratio can be varied within the limits of 1 : 2 to 1 : 4 wiffrut impairing the catalytic activity of the boron/fluorine compo~
~ In general, 0.1 to 5.0, preferably 0.2 to 4.0, % by ; weight of water, based on the amount of aniline, is used.
In a particularly advantageous variant of the proces~
according to the invention, the boron/fluorine compound, when it is employed as the catalyst for the first time, is used in the form of a homogeneous mixture, which is obtained by adding anhydrous or aqueous, preferably commercially available 40 or 70 to 75 % strength by weight hydrofluoric acid to a suspen ~ 20 ~ion of boric acid in aniline, whil~t stirring, in a tempera-; ture range o~, preferably, 50 to 80C; the addition can be started at room temperature and the heat of reaction used in order to reach and maintain the abovementioned temperature range; any excess heat of reaction must be removed by external cooling.
Of course, with this method o~ preparation of the boron/fluorine compound, according to the invention, in aniline, the de~ired amount of water, when this i8 not already obtained by the water o~ reaction formed7 can not only be introduced via the aqueous hydrofluoric acid solution but can be varied within the indicated limits by additional addition Le A 16 373 _ 4 _ ' 10~'7519 of water.
For this method oi preparation, the amount of aniline to be u~ed is sub~ect only to a lower limit, which is given by the fact that the ~mount oi aniline u~ed should be at least such that the boron/~uorine/aniline complex formed remains in solution and that the water which is additionally ; added if necessary dissolves in the aniline. The amount of aniline necessary for this depends on the corresponding solubilities and on the temperature; it can optionally be calculated from known data or determined easily by a few experiments. Oi course, it i8 al~o possible to use from the start the amount of aniline corresponding to the amount of catalyst.
The process according to the invention is illustrated below by the reaction equation:

. . .

/==\ B/F compound/H~0 f==~ /~'\

2 ~ 2 250-4000 ~ H ~ 3 ",~`
In general, the proce~is according to the invention i8 carried out in such a way that the boron/fluorine compound, according to the invention, which is ~sed,and water,are added in the chosen amountsto aniline and the reaction mixture i8 heated to the chosen reaction temperature. As described Ç above, it i8 also po3sible to prepare the boron/iluorine com-pound before the start of the reaction at a lower temperature in ~ome, or in the total amount, of the aniline. Due to the ` 25 elimination oi ammonia during the reaction, in a closed vessel the pressure rise~ above normal pressure, 80 that, in order to reach and maintain the chosen reaction pressure, it Le A 16 373 - 5 -, - . , . . .

1 0675~9 is necessary only to control the ammonia, which escepes as off-gas, by means of a control valve.
Customary apparatu~, for example autoclaves made of carbon steel, which are provided with reflux condensers and o~f-gas valves for the ammonia gas formed as well as with ~illing and emptying device~ and, if appropriate, stirring devices, can be used for carrying out the process according to the invention. Heating can also be ef~ected in the customary manner, for example by means of a heat exchange medium, directly by combustion gases or electrically.
The customary apparatus, ~or example an autocla~e cascade, can also be used for carrying out the process according to the invention continuously. The design of the apparatu~ is not a factor essential to the invention and can be in accordHnce with the state of the art. As is known, the elimination of ammonia from 2 mols of aniline, and thus the formation of diphenylamine, proceed~ at an adequate rate only until a concentration of about 50 to 55% of diphenylamine is reached in the reaction mixture. Advantageously, therefore, the reaction is then discontinued, the reactlon mlxture i8 worked up and unreacted aniline i8 r~-~mploy~d inthe reaction.
When the reaction has ended, the boron/fluorine com-pound used as the catalyst can be recovered by extracting the I reaction mixture with water. Due to the unavoidable los~es i 25 o~ water with the ammonia gas which escapes during the reac-tion, it is appropr$ate to add water in an ~mount which i8 at least æuch that the aqueous phase formed is adequate to take up the boron/fluorine compound from the reaction mixture.
i It i8 pos~ible both to employ ~o much water that the aqueous solution i8 obtained as the light phase and also to emplo~ a little water, 80 that the aqueous ~olution i8 Le A 16 373 - 6 -,, . :.: : ~,: ,. . - i.: . ..... .. .. ... . .

1067~19 obtained as the heavy phase. In the former case the aqueous solution of the catalyst complex is of low concen-tration and in the latter case it is of high concentration.
Advantageously, however, a 2-fold to 4-fold amount ~- 5 by weight of water, based on the amount of the boron/fluorine compound to be extracted, is used for the extraction. An aqueous solution of the boron/fluorine catalyst, which, with-out hesitation, can be re employed as such in the reaction and which contains the boron/fluorine compound and water in a suitable ratio, is then obtained.
The extraction of the boron/fluorine compound with water can, of course, also be carried out with any apparatus used industrially for extractions, both discontinuously and continously and in a single stage or in several stages; for example, extraction columns, separators or mixer-settlsrs can be used. In this case also the design o~ the apparatus, which is in accordance with the state of the art, is not an essential feature of the invention.
me technical advance of the process according to the invention is baqed on the following: compared with the state of the arti le~s boron/fluorine compound is requlred.
When, ~or example, 1.0% by weight of dry ammonium fluoborate was used as the sole catalyst, a conversion of 27.5% o~ the aniline employed was achieved in 7.5 hours, whilstj under the same conditions, the additional use of 2% by weight of water led, in the same time, to a conversion of 37.0~. Conversely, it was necessary, under these conditions, to use only 0.6% by weight of ammonium fluoborate in addition to 2% by weight of water in order to achieve a conversion of 27.5%
The simpler handling and eparation of the aqueous homogeneously liquid catalyst mixtures and the~ease of Le A 16 373 7 -.~

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separation thereof a~ter the reaction has ended have already been mentioned above. This is of particular advantage when the process according to the invention is carried out con-~ tinuously.
In addition, the advantage of the known use of anhydrous boron fluoride and boron fluoride complexes, that is to say the lack of a corrosive effect, is retained in full.
The corrosion characteristicsofircn(ST 37) and V4A steel (material No. 1.4571) used as the construction material was checked over a total running time of 1,520 hours by carrying out several test batches of the production, according to the invention, of diphenylamine. In detail, the follow-ing rates of corrosion were obtained,and these vary depending on whether the material came into contact with the liquid phase or mainly with the gas phase present above the latter:
liquid phase: ST 37 0.07 mm loss/year V4A (1.4571) 0.04 mm loss/year gas phase: ST 37 0.15 mm loqs/year V4A (1.4571) 0.08 mm loss/year The measured rates of corrosion are of the same orderof:
m~budeas are known for the production of diphenylamine with snhydrous boron fluoride catalysts.
Diphenylamine is useful as dye intermediate, as raw material for making rubber chemlcals, antioxidants and anthelmintic pharmazeuticals and as stabilizer for explosives.
Exam~les An externally electrically heated 5 litre steel autoclave, which was ~itted with nozzles for filling and sampling and for measuring the temperature and pressure and with a vertical air-cooled ~acketed tube for removing the ammonia ~ormed during the reaction, was used when the examples indicated below were carried out. When carrying out each of the example~, the stre~m of ammonia was uni~ormly re~ulated, by means of a valve, ~o that the temperature at the end of Le A 16 373 - 8 -.- . ~ - . . .. ... . .. .. . . .. ..

10~7519 the jacketed tube did not exceed 130C and thus entrained and gaseous aniline largely condensed in the cooled jacketed tube and refluxed; losses of aniline were thus virtually : avoided.
5 Exam~les 1 to 8 In each case, the amount of BF3 and/ where appropriate, of H20 indicated in Table I which follows were added to 3,000 g of aniline and the mixture was heated for the indicated reaction time to a reaction temperature of about 330C.
When the indicated reaction time had elapsed, the diphenylamine content in the reaction mixture was determined by gas chromatography and density measurement; it is given in % by weight, based on the reaction mixture, in Table I which follows.
Tab e I
Example Aniline BF3~ H20 Reaction DPA
time content~
(hours) %
.~
1~ 3,000 g 9.0 g = 0.3 % - 10 24.5 2 3,000 g g.o g = 0.3 % 20 g = 0.66% 10 37.0

3~ 3,000 g 12.0 g = o.4 % - 10 35.5

4 3,000 g 12.0 g = 0.4 % 30 g = 1% 10 45.0

5~ 3,000 g 19.8 g = 0.66% - 7.5 26.o

6 3,000 g 19.8 g = 0.66% 15 g = 0.5% 7.5 37.5

7 3,000 g 19.8 g = 0.66% 30 g = 1% 7.5 44.0 ` 8~ 3,000 g 39.6 g = 1.32% - 7.5 46.5 ~) Note: DPA = diphenyl~mine.
-. Because of the greater ease of metering, BF3 w~ added in the ~orm of the equi~alent amount o~ the ~olid ~F3-aniline complex, which was obtained by addi~g together equimolar ~mount~ o~ BF3 gas and aniline in ether and ~ub~equently Le A 16 373 - 9 -~06~;~S19 filtering off the complex which had precipitated.
Examples 1, 3, 5 and 8 are comparison examples.
Example 9 A mixture of 3,000 g (32.2 mols) of aniline, 14.5 g o~
73% strength by weight hydrofluoric acid (0.528 mol of HF) and 10.9 g of boric acid (0.176 mol of B), corresponding to a content of 0.4% by weight o~ BF3, based on aniline, and 0.45%
by weight of water was heated as described above to about 330C for 10 hours. The pressure generated initially was about 23 bars and this was let down uniformly in the course of the reaction until, at the end, it was 14.8 bars.
The weight of the reaction mixture was 2,890 g, with an analytically determined content o~ 42.5% of diphenyl-amine; the aniline conversion was 45.2%).
The reaction mixture was extracted by shaking with twice 300 ml of water and the aqueous phases were combined;
by careful evaporation in vacuo, 51 g o~ a 30% strength by weight fluoborate salt solution were obtained.
The organic phase was washed with sodium carbonate solution until neutral and was then sub~ected to fractional distillation; 1,155 g of diphenylamine with a boiling point of 159C/10 mm Hg and a solidification point of 52.7C were obtained (94% of theory, based on converted aniline).
Example 10 The fluoborate salt solution obtained according ~o Example 9 was added to 3,000 g of aniline and the mixture was heated, as described above, to about 330C for 10 hours.
After this time, the reaction mixture contained 50.5% by weight of diphenylamine.
Example 11 200 g of 73% strength by weight aqueous hydrofluoric Le A 16 373 - 10 -:

10675~9 acid were added to a mixture of 410 g of aniline and 120 g of boric acid, whilst stirring and cooling, at such a rate that the temperature did not rise above 75C,and the mixture was stirred until a homogeneous mixture had formed.
In each case 80 g o~ this mixture were added to, in each case, 2,950 g of aniline and the mixture was heated to about 330C in the apparatus described above.
In the first case, the initial pressure generated was 22.1 bars and this was let down uniformly to 11.1 bars during a reaction time of 5 hours. After this time, the reaction mixture contained 40.7% by weight of diphenylamine.
In the second experiment, the reaction pre~sure initially generated was 22.5 bars and this was let down uni-formly to 10.2 bars over a reaction time of 7.5 hours.
After this time, the reaction mixture contained 53.3% by weight of diphenylamine.
les 12 to 15 47.0 g of 35% strength by weigbt aqueous ~luoboric acid were added to, in each ca~e, 3,000 g of aniline and the mixture was heated to about 330C for various times in the apparatus de~cribed above.
Table II which follows gives the reaction time and the analytically determined content of diphenylamine (DPA) in the reaction mixture at the end of the reaction time.
Table II
ExampleReaction time DPA content ;~ (hour~) (% by weight) --- , _ 12 1.5 15.5 13 2.5 ~2.3 14 4.5 35.5 5.5 46.o Le A 16 373 -; ~
- . .. -. . , . - . . ... -.
.

10~751~

Examples 16 and 17 (Compari~on Example~) In each case 47.0 g of aqueous fluoboric acid were mixed with, in each cage, 100 g of aniline and the water was distilled from the mixture in vacuo. In each case 17.0 g of anhydrous HBF4 in aniline were thus obtained and, after adding further aniline until the total amount wa~
3,000 g, the mixture was heated to about 330C for various times in the apparatus described above. Table III which follows gi~es the reaction times and the diphenylamine (DP~) content in the reaction mixture at the end of the re~ction time.
Table III
Example Reaction timeDPA content ,'! (hours)(% by weight) 16 6 27.3 17 7.5 ~1.0 ,.,~
Example 18 3,000 g of aniline, together with 30.0 g of finely ~ powdered a3monium fluoborate and 30 g of w~ter, were heated to ; about 330C for 7.5 hours in the apparatus described above.
At the end of the reaction time, the diphenylamine content in the reaction mixture was 40.5% by weight.
Exam~le 19 (Comparison Example) 3,000 g of aniline, together with 30.0 g of finely powdered ammonium fluoborate, were heated to about 33~C for 7.5 hours in the apparatu~ described above. At the end of the reaction time, the diphenylamine content in the reaction mixture was 30.0X by weight.
; Exam~le 20 3,000 g of aniline, together with 39.6 g (1.32% by ` 30 weight) of boron tri~luoride (added in the form o~ 95.5 g o~
- Le A 16 373 - 12 _ . .

aniline/BF3 complex) and 30 g (1.0% by weight) of water, were heated to about 310C for 9 hours, whilst stirrlng, in the apparatus described above. By controlling the let-down valve, the ammonia formed was released at such a rate that the pressure fell from an initial value of 16.8 bars down to 14.5 bars towards the end of the reaction time. The analytically determined diphenylamine content was then 38.5% by weight.
ample 21 3,000 g of aniline, 9.0 g (0.3% by weight) of boron trifluoride (added as 21.5 g of aniline/BF3 complex) and 10.0 g ~0.33% by weight) of water were heated to about 350C for 7 hours, whilst stirring, in the apparatu~ described above.
The pressure initially generated was 23.3 bars and this was let down uniformly to 18.0 bars during the reaction time.
After this time, the reaction mixture contained 41.5~ by weight of diphenylcmine.
Exam~le~22 (Com~arison Exam~le~
13.0 kg of aniline, to which 100 g (0.77% by weight) of '~ dry, finely powdered a~monium fluoborate had been added, were heated to about 330C, whilst stirring, in a 20 litre auto-~, clave, which can be heated electrically and whlch is fitted i with a stirrer, a thermometer, a pressure gauge and sampling branches and with a column attachment with a cooling device and an off-gas valve. By regulating the let-down valve, the i .
stream of ammonia was so ad~u~ted that the reflction pressure .1 fell from 14.8 bars at the start of the reaction down to 11.9 bars at the end of the reaction time. Samples were taken at the interYals indicated in Table IV which follows and the diphenylamine content w~s determined.

`~1 ~ Le A 16 373 - 13 -', :

10~;7519 Table IV
._ _ Time (hours) DPA content (% by weight) O
2 10.0 4 18.5 7 29.0 9 35.0 Example 23 13.0 kg of aniline, to which 100 g (0.77% by weight) of dry, finely powdered ammonium fluoborate and 50 ml (0.38%
by weight) of water had been added, were heated to about -330C, whilst stirring, in the apparatuq described in Example 22. By regulating the let-down valve, the ammonia formed was released at such a rate that the pres~ure fell from 15.3 bar~ at the start of the reaction down to 9.0 bars at the end of the reaction time. Samples were taken at certain intervals and the diphenylamine content in the reac-tion mixture was determined analytically. The corresponding values are summarised in Table V.
Table V
,, ~
Time (Hours) DPA content (% by weight) . O
3 17.5 27.0 ~ 25 7.5 i 35.5 `1 10.5 43.0 ~ 13 52.5 q ~
.; 13.0 kg of aniline, to which 117 g of 73% strength by ; 30 weight hydrofluoric acid, 86 g o~ boric acid and 24 g of water had been added, were heated to about 330C, whilst Le A 16 373 14 ,, . -. , ~
.: - , - - - . . . ~, .. . .

stirring, in the apparatus described in Example 22; the addition corre~ponded to 95 g (= 1 mol %) of BF3 and 1~0 g (= 1% by weight) of H20. By regulating the let-down valve, the ammonia formed ~as let down at quch a rate that the pressure in the autoclave ~ell from 19.0 bars at the start of the reaction down to 10.8 bars at the end of the reaction time.
Samples were taken at certain intervals and the diphenylamine content of the reaction mixture was determined analytically.
- The correqponding values are summarised in Table VI which 10 follows.
Table VI
Time (hours) DPA content (% by weight) O
3 20.5 6 34.5 9 45.0 ` 12 52.0 13.5 57.5 For illustration, the comparable results from Examples 22, 23 and 24 are plotted as a graph in Figure 1.
Ex~a~_2~ (In this Example "parts" are parts by welght).
- Analogously to Example 11, a homogeneous catalyst mixture is prepared by mixing 200 parts of 73% strength by , . . .
weight aqueous hydro~luoric acid and 120 parts of solid boric acid in 400 parts of aniline at a maximum of 75C; according to calculation, the mixture then contains 163.5 part3 o~ the bor~n/fluorine compound with a molar ratio of boron to fluorine of 1 : 3.57 and 156 parts of water.
.i 3 For an autocla~e batch, thi~ mixture iY added to the anlline employed in an amount such that the concentration of the boron/~luorine compound is 0.78% by weight and the Le A 16 373 - 15 -: .

concentration of water is 0,74% by weight, based on the amount of aniline employed.
The mixture is then heated, as de~cribed above, to 330 to 350C, the ammonia formed being relea~ed a~ a ga~, and the reaction i~ di~continued a~ter about 55% by weight of the aniline employed has been converted to diphenylamine. The pressure is then about 9.8 bars and the diphenylamine content in the reaction mixture is about 49.5% by weight.
me contents of the autoclave are let down, via a cooler, into a receiver and extracted, at about 70 to 80C, with an aqueou~-extraction ~olution from a preceding batch (obtained as described below) and fre~h water in a ratio of . 100 : 10 : 1.
The mixture from the extraction then separates into a light crude oil phase containing le~s than 0.1% by weight of the boron/fluorine compound and a heavier aqueous phase containing about 23% by weight of boron/fluorine compound, the latter being the socalled extraction solution.
The crude oil phase i~ extracted, for neutralisation, by shaking with 10% ~trength by weight aqueou3 sodium carbonste ; 20 solution and then, by fractional distillation, separated into diphenylamine and aniline, which i~ re-employed for the reaction. This gives diphenylamine wlth a ~olidification ~; point of 52.8C in a yield of 96% o~ theory, based on converted aniline.
Exam~le 26 ~ For an autoclave batch, the aqueoua extraction solu-t~ tion, obtained as described in Example 25, i8 added, as the catalyst, to the aniline employed in an amount ~uch th~t the content o~ the boron/fluorine compound in the reaction mixture i8 0.6% by weight and the water content is about 2% by welght, in each case based on the aniline employed. The Le A 16 373 - 16 -mixture is heated, ~8 described ~bove, to 330 - 350C, the ammonia formed being released as a gas, and the reaction is discontinued after about 55% by weight of the aniline has been converted to diphenylamine.
The contents of the autoclave are then let down, as described in Example 25, via a cooler into a receiver and extracted with aqueous extraction solution and fresh water.
Further working up is also carried out as described in EXample 25. This gives a diphenylamine of the same quality, in a comparable yield.
~' , .
. j . .

; Le A 16 373 - 17 -~; . .

- .:, . . . . .. : . : . . .. . . . . .... . . . .

Claims

WHAT IS CLAIMED IS:

1. Process for producing diphenylamine which comprises heating aniline under pressure to temperatures of 250 to 400°C. in the presence of a boron/fluorine compound and water.

2. Process of claim 1 carried out under a pressure between 6 and 35 bars.

3. Process of claim 1 carried out at the boiling point of the reaction mixture at a temperature between 300 and 360°C. and at pressures between 12 and 30 bars.

4. Process of claim 1 wherein the boron/fluorine compound is present in an amount of 0.1 to 5.0% by weight, calculated as BF3 and based on the weight of aniline used.

5. Process of claim 4 wherein 0.2 to 3.0% by weight of the boron/fluorine compound, calculated as BF3 and based on the weight of aniline used, is used.

6. Process of claim 1 wherein water is present in an amount of 0.1 to 5,0 by weight, based on the weight of aniline used.

7. Process of claim 6 wherein water is used in an amount of 0.2 to 4.0% by weight, based on the weight of aniline used.

8. Process of claim 1 wherein the boron/fluorine compound is boron trifluoride, ammonium fluoborate or anilinium fluoborate.

9. Process of claim 1 wherein a mixture of hydro-fluoride with boric acid or boron trioxide in a molar ratio of boron : fluorine of between 1 : 2 and 1 : 4 is used as the boron/fluoride compound.

10. Process of claim 8 wherein anhydrous or aqueous 40, or 70 to 75% by weight strength by weight hydrofluoric acid is added with stirring to a suspension of boric acid in aniline at a temperature range of 50 to 80°C.

11. Process of claim 1 wherein the boron/fluorine compound is separated off from the reaction mixture when the reaction has ended by extracting with water and the extract thus obtained is re-employed in a further reaction as claimed in claim 1 as the mixture of boron/fluorine compound and water.